The present invention relates to nucleic acids encoding inhibitors of fungal polygalacturonases. The nucleic acids are used to produce fungus-resistant transgenic plants.
Control of fungal plant pathogens is of major economic importance. For instance, extensive efforts have been focussed on controlling fungal rot in fruits. Previous research has involved hygienic and chemical means to control the fungi (Raese, Hort. Rev 11:357 (1989) and Pimentel et al., BioScience 42:750 (1992)). Some reduction in fruit rotting has been achieved by careful handling of picked fruit, by hygienic washing, by reducing the storage temperature of the ripening fruit, breeding fungus-resistant cultivars and by the application of effective fungicides. Little success has been achieved by breeding programs to select for fruit that are more resistant to fungi such as Botrytis cinerea, Glonierella citigulata, and Alternaria alterizata.
Recently, research has focused on the plant's response to attack by fungi, centering on induced enzymatic activities such as chitinases, which can attack fungi directly (Meins & Ahl, Plant Science 61:155 (1989)). Infection by the fungal pathogen, Cercospora nicotianae, was partially reduced in tobacco plants expressing large amounts of tobacco chitinase gene (Neuhaus et al., Plant Mol. Biol. 16:141 (1991)). In addition, other genes for enzymes in induced plant defense pathways, such as phytoalexin biosynthesis, have been introduced into transgenic tobacco plants and have been shown to partially retard the invasion by the fungal pathogen, B. cinerea (Hain et al., Nature 361:153 (1993)).
Fungal pathogens invade plant tissues by the secretion of enzymes which degrade components of the plant cell wall. A major class of enzyme secreted by fungal plant pathogens are polygalacturonases which degrade pectic cell wall components. One of the ways in which plants combat fungal infection is by the production of polygalacturonase inhibitor proteins (PGIPs) (Albersheim & Anderson, Physiol. Plant Path. 2:339 (1972); Fielding, Gen. Micro. 123:377 (1981); Abu-Goukh et al., Physiol Plant Path. 23:111 (1983); Hahn et al., in T. Kosuge & E. Nester, Plant Microbe Interactions, Vol. 3, (1989)).
The nucleic acid sequence of a PGIP from Phaseolus vulgaris has been previously reported (Toubart et al., Plant J. 2:367, 1992). This PGIP is expressed at low levels throughout the bean plant and is a non-competitive inhibitor of polygalacturonase from Colletotrichum lindemuthianum (De Lorenzo et al., Physiol. Mol. Plant Path. 36:421 (1990)). PGIP from pears is a competitive inhibitor of polygalacturonase from B. cinerea (Abu-Goukh et al., Physiol. Plant Path. 23:111 (1983)) and is specifically expressed at high levels in the pear fruit.
Despite efforts to breed fungus-resistant plants, the prior art has yet to provide effective means of controlling a number of fungal infections in fruit. The present invention addresses these and other needs.